Extraction

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EXTRACTION OF GEOTHERMAL ENERGY AND ELECTRIC POWER GENERATION USING
A LARGE SCALE HEAT PIPE
Shunji Kusaba
1
, Hirosi Suzuki
1
, Kazuo Hirowatari
2
,
Masataka Mochizuki
3
, Koichi Mashiko
3
, Thang Nguyen
3
, and Akbar Akbarzadeh
4
,
1
Research Laboratory, Kyushu Electric Power Co., Inc., 2-1-47, Shiobara, Minami-ku, Fukuoka, 815-8520, Japan,
2
Geothermal Department, West Japan Engineering Consultants, Inc., 2-1-82, Watanabe-Dori, Chuo-ku, Fukuoka, 810-0004, Japan,
3
Thermal Engineering Department, Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan
4
Department of Mechanical & Manufacturing Engineering, Royal Melbourne Institute of Technology,
Bundoora East Campus, PO Box 71 Bundoora 3081, Victoria, Australia
Key Words: geothermal heat extraction, heat pipe, turbine
ABSTRACT
In order to utilise low enthalpy natural heat sources, a heat
pipe using a binary fluid is a good device which can extract
heat without using electric power. When the heat flux in a
geothermal bore is moderate, a long heat pipe is needed. We
developed a 150 m long, large scale heat pipe of 150mm
outer diameter, in which liquid feeding tubes with showering
nozzles were installed. It has an excellent thermal
performance compared with other long vertical evaporators.
A demonstration test was carried out in a well with
temperatures between 100 and 150
o
C at a geothermal site in
Kyushu, Japan. The heat pipe extracted 90 kW heat at a
working temperature of 80
o
C. We also calculated the
extraction rate for different lengths of the heat pipe and
different temperatures of the geothermal source. Similar
pipes can be used for heat extraction of high temperature
geothermal fluids .
In addition to heat extraction, we also studied the
performance of a turbine which can be installed on top of a
heat pipe using a binary fluid. We used a modification of a
thermosyphon, with excellent heat and mass transfer
characteristics, and incorporated a turbine. The heat transfer
in a thermosyphon occurs through the circulation of a binary
working fluid through a sequence of evaporation, vapour
transfer, condensation and liquid return. It is then possible to
convert some of the energy of the vapour stream into electric
energy by installing a turbine which is coupled to an electric
generator. Simulation showed that a heat pipe turbine,
using R-123 as working fluid, can generate about 7.8 kW
electric power at a turbine speed of 3000 rpm when the rotor
diameter is about 0.8m and the evaporator section of the heat
pipe is 150m long. The Heat Pipe Turbine or Thermosyphon
Rankine Engine (TSR) is therefore an interesting concept for
power generation using geothermal or other low grade heat
sources.
1. INTRODUCTION
Utilisation of unused thermal fluids with rather low
temperature, such as geothermal waste heat, hot spring
water, but also urban waste heat, has been looked at in
recent years. As a heat transfer device for otherwise unused
thermal fluids, a high performance heat pipe is attractive.
However, substantially high performance can not be expected
simply by enlarging the diameter and length of a standard
heat pipe. In a conventional thermosyphon or heat pipe
consisting of a single tube, thermal performance is restricted
by entrainment and flooding phenomena. Furthermore, it is
difficult to maintain a uniform liquid film throughout a large
evaporator which causes the heat transfer performance to
deteriorate.
In order to improve the thermal performance, we have
developed a large scale, loop type and gravity assisted heat
pipe where vapour- and liquid flow passages are separated
by installing liquid feeding tubes with showering nozzles
inside the evaporator. We have conducted a demonstration
test of such a heat pipe at a geothermal site..
2. DEMONSTRATION TESTS OF EXTRACTING
GEOTHERMAL ENERGY WITH A HEAT PIPE
A heat pipe is a heat transfer device which can transfer
quickly heat from a high temperature section to a low
temperature section. Heat pipes are presently used for
cooling electronic components, snow melting systems, etc.
The heat transfer rate of a simple heat pipe has limitations.
In order to overcome these limitations, we developed several
loop type structures. We used for initial tests several heat
pipes such as a 12 m long, vertical pipe with 46 mm outer
diameter and a 70 m long, inclined pipe with 150 mm outer
diameter. These types have an up to ten times greater heat
transfer rate than a conventional heat pipe. The merits of
extracting geothermal heat by using a heat pipe are :
1) No power is needed for extracting and transferring
geothermal heat.
2) Even in a dry geothermal well filled with hot air or
vapour, a heat pipe can still extract some heat.
3) A loop type heat pipe can control the heat transfer rate by
controlling the flow rate of the returning working liquid.
A schematic diagram of our loop type heat pipe with
showering nozzles is shown in Fig.1 (Mochizuki et al. 1994).
The liquid feeding tubes with a thermal insulator are
installed inside the evaporator to create a one way flow loop.
Showering nozzles are attached along the liquid feeding tube
at a specified spacing to allow uniform spurting of the
working liquid. Furthermore, a control valve has been
installed at the bottom of the condenser to adjust the flow
rate of the working fluid. The corrugated shaped tube with
anti-corrosion material was used as an evaporator container.
Geothermal heat extraction was carried out in a well at a
geothermal site in Kyushu, Japan, using a 150 m long heat
pipe. Before operating the heat pipe, the temperature
distribution of the geothermal well was measured;
3489


Proceedings World Geothermal Congress 2000
Kyushu - Tohoku, Japan, May 28 - June 10, 2000
Kusaba
temperatures at depths from 70m to 150m were between 120
and 150
o
C. The heat pipe extracted heat at a rate of 90 kW
at a working temperature of 80
o
C. This test was sponsored
by MITI in cooperation with Kyushu Electric Power Co. We
simulated similar tests for different lengths of the heat pipe
and for different temperatures of geothermal sources.
3. PREDICTION OF PERFORMANCE OF HEAT
PIPES
On the basis of the demonstration test at the Kyushu
geothermal site, we tried to predict heat extraction of larger
scale heat pipes. Fig.3 shows the results for different fluid
temperatures (hot water) inside a well. In Fig.4, results are
shown assuming geothermal fluids exist in the well and that
their temperature is either 100 or 150
o
C; at the higher
temperature, 350 kW heat could be extracted using a 300 m
length evaporator heat pipe.
It is suggested that a similar type of heat pipe can be used to
extract heat from higher temperature geothermal fluids and
other heat sources.
4. ELECTRIC POWER GENERATED BY A HEAT
PIPE TURBINE
4.1 Heat Pipe Turbine
A Heat Pipe Turbine is a new concept for power generation
from solar, geothermal or other available low grade heat
sources. It uses a modification of the thermosyphon cycle,
which has excellent heat and mass transfer characteristics,
and incorporates a turbine in the adiabatic region. The
configuration involves a closed, vertical cylinder functioning
as an evaporator, an insulated section and a condenser
(shown in Fig.5). The turbine is placed near the upper end
between the insulated section and a condenser section; a
plate is installed to separate the high pressure region from
the low pressure region in the condenser. Conversion of the
fluid enthalpy to kinetic energy is achieved through a nozzle.
The mechanical energy developed by the turbine can be
converted by direct coupling to an electrical generator.
The working fluid is located in the lower evaporator end of
the heat pipe and flows to the upper region after evaporation.
The condenser is situated in the upper region. Here the
vapour changes into liquid again and the condensed liquid
returns to the evaporator by gravity. Between the evaporator
and the condenser, the vapour passes through the turbine.
Akbarzadeh et al. tested the performance of a heat pipe
turbine. In their prototype, a reaction turbine was
introduced. The diameter of the heat pipe was 160 mm and
its height 3.15 m. An electrical power output of 5.5 W at
4788 rpm was obtained from a heat input of 4.4 kW. Another
prototype consisted of a cylinder of 2.8 m height with a
diameter of 0.5 m.. The heat input was 10 kW and an
electrical output of 0.1 kW was obtained at 6000 rpm.
The thermodynamics of such a heat pipe turbine is discussed
below.
4.2 Thermodynamics of a heat pipe turbine
Let us assume that Q w

( ) is the rate of heat transfer in the
evaporator section. This heat can be provided by an external
source, such as solar, geothermal, waste, etc. The rate of
mass flow of vapour m kg s

( / ) generated in the evaporator
is then:
m
Q
h
fg


· , (1)
where hfg (J/kg) is the change in specific enthalpy from
saturated liquid to saturated vapour in the conditions of the
evaporator.
Now let us consider the configuration of the reaction turbine
presented in Fig. 4. Assuming that the rotor has a radius of
R(m) with a total nozzle exit area of Ao(m
2
) and the angular
velocity of the rotor is ω (rad/s), then:
U=Rω, (2)
where U is the velocity at the nozzle.
If Vr (m/s) is the relative velocity of vapour leaving the nozzle
with respect to the nozzle, the absolute velocity of Va (m/s)
of the vapour with respect to a stationary observer will be
given by:
Va= Vr - U, (3)
Conservation of mass yields the relation:
m V A
r

·ρ
o o
, (4)
where ρ
o
(kg/m
3
) is the density of the vapour leaving the
nozzle. The torque T (Nm) produced by the flow of the
vapour through the nozzle is given by:
T m V R
a
·

. (5)
The produced power W

can then be calculated as:
W T m V R
a
• •
· · ω ω . (6)
If one assumes an isentropic efficiency k for the adiabatic
expansion of the vapour from the condition existing in the
evaporator to the condition at the nozzle outlet, then
conservation of energy yields:
W m k h h m V
i a
• • •
· − − ( )
o
1
2
(7)
where hi (J/kg) and ho (J/kg) refer to the specific enthalpy of
the working fluid at the evaporator and nozzle outlet
respectively.
After elimination of the parameters m V V
r a

, , , ω and R in
the above equations, it can be shown that the following
relations exist:
( )
W
Q
h
fg
Q
h
fg
r A
k h
i
h

• •
· − −
¸
¸


_
,


o o
o
2
2
*
Q
h
fg
r
o
A
o
Q
h
fg
r
o
A
o
k(h
i
h
o
)
• •
¸
¸


_
,


¸
¸


_
,



¸


1
]
1
1
− − −
2
2
(8)
and
3490
Kusaba
( ) U m
Q
h A
k h h
fg
i
·
¸
¸


_
,


− −


ρ
o o
o
2
2 . (9)
As an alternative equation for output power the following
relation can also be derived:
( ) ( )
( ) ( )
W A U
U U k h
i
h U k h
i
h
·
− + + − + −
ρ
o o
o o
2 2
2 2
(10)
By examining equation (9), it can be seen that:
A
o

( )
Q
h k h h
fg i

− ρ
o o
2
. (11)
The above relation puts a restriction on the maximum of the
total nozzle exit area. This means that for a certain condition
in the evaporator and condenser region, in order to be able to
generate power the above restriction should be observed. The
equal sign in the equation above relates to the condition of a
stationary nozzle (U=0) when no power is produced.
Examination of equations (8) and (9) shows that as Ao
decreases Wo , U increases. This means that for the same
rate of heat input to the evaporator, faster machines require
less nozzle exit area and at the same time they produce more
power. The theoretical limit for maximum power which can
be produced can be obtained from Equation (8) by finding
the limit when Ao → 0. This implies that:
( )
( )
W
Q k h h
h
i
fg


·

max
o
(12)
and relates to the turbine rotating at infinite speed.
From the above equation, the maximum efficiency can be
obtained :
( )
η
max
max
· ·
− W
Q
k h h
h
i
fg
o
o
. (13)
For k=1, the above ηmax will be the Rankine Efficiency.
4.3 Predicted Electric Output of Heat Pipe Turbine
when using Geothermal Energy
We also estimated the predicted output of a heat pipe
turbine. Several types of working liquids were considered.
Finally, we selected R-123 as working fluid because of its
high performance efficiency. The pressure performance is
good for rotating the rotor of the heat pipe turbine. In Fig. 6,
the predicted performance is shown for temperatures of 84
o
C at the evaporator and 36
o
C at the condenser by using
R123 as working fluid. At a 3000rpm speed of the rotor, an
electric output of 7.8kW has been predicted by utilising
100kW of heat extracted by the heat pipe.
5.CONCLUSION
A heat pipe is an effective heat extraction device which
can utilise heat from vacant wells. By using a large scale
heat pipe of 300m length, it is predicted that 350kW of
geothermal heat can be extracted from a geothermal well
filled with fluids of 150
o
C. For a more practical use of this
heat, we have shown that electric power can be generated by
a heat pipe turbine and that it is possible to generate, for
example, 7.8kW electric power from 100kW heat extracted
by a heat pipe.
REFERENCE
M. Mochizuki et al. (1994). A Performance of Loop-type
Heat pipe having Showering Nozzles. Proceeding of 8
th
IHPC, pp. 448-451.
Fig.1: Basic structure of a loop type heat pipe having
nozzles.
L i q u i d f e e d i n g t u b e
c o v e r e d w i t h t h e r m a l
i n s u l a t o r
S p i r a l w i c k f o r
d i s t r i b u t i o n a n d
h o l d i n g l i q u i d
E v a p o r a t o r
S h o w e r i n g
n o z z l e s
C o n d e n s e r
C o n t r o l v a l v e
V a p o r f l o w
3491
Kusaba
Fig.2: Demonstration equipment used to extract geothermal
heat from a well using a large heat pipe
P u mp s f o r
c i r c u l a t i n g
l i q u i d
Re s e r v o i r
G L
1
5
0
m
7
0
m
p u m p
( 1 9 8 m m )
He a t e x c h a n g e r
C o n d e n s e r
Co n t r o l
V a l v e
Tv
Tc
T h
W a t e r l e v e l
A l a r g e s c a l e l o o p t y p e
h e a t p i p e ( D 1 5 0 mm x L 1 5 0 m)
T h
T h
3492
Kusaba
Fig. 3: Calculated results in case of a well filled with hot water
Fig. 4: Extracted heat as a function of the length of the heat pipe
0 100 200 300 400 500 600
0
200
400
600
800
1000
Length of heat pipe evaporator (m)
E
x
t
r
a
c
t
i
o
n

h
e
a
t

(
k
W
)
Tg=100 C
Tg=150 C
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0
0
2 5
5 0
7 5
1 0 0
1 2 5
1 5 0
1 7 5
2 0 0
2 2 5
2 5 0
Saturated temperature of geothermal (C)
E
x
t
r
a
c
t
i
o
n

h
e
a
t

(
k
W
)
t e m p . o f C o o l i n g Wa t e r T c = 3 0 ºC
3493
Kusaba
Fig. 5: Schematic diagram of a heat pipe turbine
Fig.6: Predicted output of a heat pipe turbine
P o we r v s . S p e e d
0
2 0 0 0
4 0 0 0
6 0 0 0
8 0 0 0
1 0 0 0 0
1 2 0 0 0
1 4 0 0 0
1 6 0 0 0
1 8 0 0 0
2 0 0 0 0
0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0 8 0 0 0
S p e e d ( R P M)
P
o
w
e
r

(
W
)
3494

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